The present invention proposes a method and an equipment for the automatic sorting of objects, such as mailed parcels.
Conventional sorting machines capable of the highest productivities, are basically divided into three types, i.e., tilt trays, shoe sorters, and cross belts. The type used depends on the solutions adopted for the unloading of the objects to the destination.
With tilt trays: the conveyor unit, usually comprising a tray of a material with low friction mounted on a truck, is turned over at the destination so that the object on board is accelerated laterally by the gravity force, and reaching then the destination.
With shoe sorters: a succession of conveyor units, dragged by chains forms a conveyor belt; each conveyor unit comprises a shoe sorter element which can be controlled to move through the conveyor platform in a direction orthogonal to the advancing direction of the machine, to push the object to the destination.
Usually this equipment is fitted with very short conveyor units so that it is possible to apply to each object to be sorted a number of shoe sorters which, when combined are able to accommodate the objects length. This is a very useful solution to optimize the productivity when objects having different dimensions are to be sorted.
With cross belts: a sorting machine is formed by a series of trucks fitted with conveyor belts, the conveyor and sorting elements, oriented perpendicularly with respect to the direction of the sorting machine, which are actuated for the unloading in order to direct the object towards the collecting means at the sides of the route.
The cross belt equipment has reached considerable levels of sophistication since the belt unloading has very precise control functions. Sorting machines are known, in which each object is measured, in order to be loaded exactly in the middle of the belt whereupon its correct positioning is checked, and controlling, when necessary, the movement of the belt to correct the centering.
When unloading, the control actuates the belt with leads and acceleration ramps which are a function of the advancing speed, or of the object's position on the belt, or of the object's dimension, or sometimes the kind of the object, in order to assure the greatest unloading precision even though very different objects are being handled.
The above-described equipment is described, for example, in the European Patents No. 0 481 341, No. 0 518 180 and No. 0 556 866, to which reference is made for more details.
All the known types of equipment have limited productivity. That is, the number of objects that the machine is capable of sorting in a time interval is limited, as determined essentially by the fact that the advancing speed of the machine must be lower than 2 m/sec in the greatest part of the facility. At this speed, a sorting machine comprising, for example a continuous succession of units having a pitch (spacing) of 500 mm, will drive 14400 units per hour to an object loader, and will sort as many objects per hour, provided that any unit is capable of receiving and sorting an object during any revolution of the machine.
For small objects to be sorted (dresses, books, envelopes, little parcels) the above-mentioned value represents virtually the actual technological limit. More elevated speeds of the sorting machine would involve longer transients of acceleration during the loading with greater problems of precision due to the sliding and rolling effects of the objects.
Furthermore speeds near 2 m/sec may cause effects that compromise the reliability of the treatment of light-weight objects, due to the braking action of the air resistance, these effects getting rapidly worse with the increasing of the speed.
The productivity limits of the existing sorting techniques make their use difficult or impossible in certain applications. That is, the concentration of the objects in some facilities may require the automatic sorting of object flow rates higher than those obtainable by known sorting machines, and towards a greater number of destinations.
In order to satisfy these requirements numerous solutions have been realized using the available machines, but, with results not very satisfying because of complexity and high prices.
In order to double productivity, a first previously-proposed solution involves the use of two sorting machines moving along parallel routes and unloading the objects towards common exits. (It is the case, for example of two superposed sorting machines.)
That solution is however very expensive because it requires two complete machines, more space, and also because the sorting towards common exits creates new complexities, for example the necessity of introducing conveyor belts and controls at the destination in order to drive both machines with order and without crashes, to sort the objects at the destination. Therefore, this solution is suitable only in the case of applications with high productivity and a low number of destinations.
Another technique, largely used when the application involves a high number of destinations and a productivity higher than that of a single sorting machine, consists in the use of so-called "presorting". That technique involves the use of two machines in parallel and the division of the number of exits between these two machines. Basically, one of the machines will unload at half of the exits and the other machine will unload at the other half of the exits.
In practice, it is necessary to divide the entering objects in two principal directions, each direction having half of the final destinations. Since downstream of the presorting station two sorting machines are used, each one with half of the total number of destinations, it will be possible to obtain the required productivity provided that the two flow rates of the presorted objects are maintained constantly equal.
This condition is in reality very onerous and involves many complexities and costs both for dividing the entering objects, and for realizing suitable accumulations of objects presorted so that to reduce unbalancements between the two flows directed towards the sorting machines.
Even if it can be assumed that the destinations of the two groups are equiprobable, during the presorting in a short period, the objects are not usually divided equally between the two directions.
Without suitable dynamic accumulations these imbalances would constantly produce a loss of productivity because if one machine became saturated, it would slow the flow of entering objects, while the other machine would have an incompletely used production capacity.
Therefore it is possible to understand how sorting machines can encounter very expensive costs, and great complexity when efforts are made to increase productivity.